Simulating Aerosol Optical Depth and Direct Radiative Effects over the Tibetan Plateau with a High-Resolution CAS FGOALS-f3 Model

Current global climate models cannot resolve the complex topography over the Tibetan Plateau (TP) due to their coarse resolution. This study investigates the impacts of horizontal resolution on simulating aerosol and its direct radiative effect (DRE) over the TP by applying two horizontal resolutions of about 100 km and 25 km to the Chinese Academy of Sciences Flexible Global Ocean-Atmosphere Land System (CAS FGOALS-f3) over a 10-year period. Compared to the AErosol RObotic NETwork observations, a high-resolution model (HRM) can better reproduce the spatial distribution and seasonal cycles of aerosol optical depth (AOD) compared to a low-resolution model (LRM). The HRM bias and RMSE of AOD decreased by 0.08 and 0.12, and the correlation coefficient increased by 0.22 compared to the LRM. An LRM is not sufficient to reproduce the aerosol variations associated with fine-scale topographic forcing, such as in the eastern marginal region of the TP. The difference between hydrophilic aerosols in an HRM and LRM is caused by the divergence of the simulated relative humidity (RH). More reasonable distributions and variations of RH are conducive to simulating hydrophilic aerosols. An increase of the 10-m wind speed in winter by an HRM leads to increased dust emissions. The simulated aerosol DREs at the top of the atmosphere (TOA) and at the surface by the HRM are -0.76 W m(-2) and -8.72 W m(-2) over the TP, respectively. Both resolution models can capture the key feature that dust TOA DRE transitions from positive in spring to negative in the other seasons.

Automated summary

Current global climate models with coarse resolution cannot accurately simulate the complex topography over the Tibetan Plateau. This study shows that a high-resolution model performs better in reproducing the spatial distribution and seasonal variations of aerosol compared to a low-resolution model. The fine-scale topographic forcing, such as in the eastern marginal region of the Tibetan Plateau, cannot be accurately simulated by a low-resolution model. Increasing the 10-m wind speed in winter leads to increased dust emissions. The aerosol direct radiative effects at the top of the atmosphere and at the surface over the Tibetan Plateau are -0.76 W m(-2) and -8.72 W m(-2) respectively.